Known control methods for controlling the power of a regenerative type Stirling engine do so by changing the mean pressure prevailing in the working chambers of the engine, such engine typically having a hot chamber and a cold chamber per cylinder, these being separated from one another and adapted to be alternately reduced and enlarged in volume by a piston movable in the cylinder. The hot chamber is connected to the cold chamber within the same engine cylinder or to a cold chamber in another cylinder (operating in a phase-displacement manner) by way of a flow path having a regenerator and cooler therein.
To control power, the mean pressure prevailing in the working chambers is so modified that a high pressure is present in the chambers at a high engine torque demand and a low pressure at a low torque demand. These pressure levels, as well as varying intermediate levels, are achieved by means of a compressor driven by the engine and which is effective to pump the working medium into a reservoir. In the case of a power reduction, the reservoir is maintained at a typically high pressure. A compressor for this task has to meet very high standards. It must have a high pressure ratio, must operate without lubrication of the piston and must be sealed to prevent the escape of hydrogen. These requirements can be met only with difficulty, if they are met at all, and only at great expense. Such compressors may be separate units or may be extensions of the piston extending into close-fitting auxiliary cylinders. The piston extensions may be one or more in number and usually extend from the bottom side of the principal piston. In addition to the increased complexity and cost of utilizing a system which is compressor actuated to transfer gases to or from the working chambers to a reservoir, there is the additional problem that pumping of the working medium out of the working chambers by the small compressors takes place relatively slowly.
Separate small compressors have become a popular means of implementing mean pressure control which in turn provides torque control for the engine. Mean pressure control systems of the prior art have emphasized the need for equalizing the mean pressures in the different working chambers, separated by double acting pistons. However, such prior art systems employ injection or ejection of high pressure from one working chamber at a time which creates a temporary inequilibrium lasting for three or four cycles of the engine until mean pressures stabilize again. What is needed is a mean pressure control system which eliminates independent compressors and yet provides a temporary inequilibrium in mean pressures during a torque demand change commensurate with the inequilibrium now experienced by prior art systems.